1. Field of the Invention
The invention relates to fish screens for screening fish from water intakes for various installations such as pumps, canals and ditches, generators, water diversion structures and the like.
2. The Prior Art
The U.S. Government has mandated that fish screens be installed in many waters within the United States. For example, Public Law 102-575, Title 34, Section 3406(b)(21), directs the Secretary of the Interior to "avoid losses of . . . anadromous fish resulting from unscreened . . . diversions on the Sacramento . . . river . . . and Sacramento-San Joaquin Delta . . . " Such screens are also used by private industry and others to protect fish.
Many different types of fish screens have been produced and installed over the past 20 years (and before). Nearly all of these have been custom designs for specific sites and applications. Existing screening systems are expensive because, among other reasons, these systems rarely are, and generally cannot be, mass produced. The majority of these screens are river bank designs using flat plate screens or drum screens and have problems balancing flows through and past the screens, handling debris that collect on the screen, bypassing fish and preventing predation by other fish. In the latter regard, predator fish typically congregate in the slower backwash areas near prior art screening devices and near outfall structures where juvenile fish are returned to the river.
More recently, some screen systems have been located on the river bottom and use an air or water backwash arrangement to prevent debris buildup. These screen systems, which, as will appear, are perhaps closest in concept to the Screen system of the present invention, are generally cylindrical in shape and are attached, by bolting, to an underwater piping system that conveys filtered water to the pump or siphon of the water diversion structure. Such cylindrical river bottom screen systems suffer a number of important disadvantages. For example, because the systems are anchored, divers are required to install, remove, maintain and/or inspect them. Further, the air cleaning systems associated with the screens must act in all directions (up and down) thereby producing less jetting action and less effective cleaning of the bottom screen surface. Further, some arrangements provide a habitat for predator fish. In addition, balancing flows through multiple screens is often difficult because variable river conditions change the flow patterns. Also, cylindrical screens are to some extent limited to smaller installations because the complexities in cleaning, handling, and operation increase with size.
More generally, existing fish screen systems use both large screen and small screen approaches. Basically, large screen system use either drum screens or vertical wedge-wire screens and each has particular disadvantages. For example, drum screens require a regulated water surface elevation and permit passage of debris that must be handled eventually. Further, the seals are difficult, the mechanical elements require extensive maintenance and a low percentage of the screen is actually exposed to flow. Further, such drum screens are prone to substantial siltation because of the low through-screen velocities. In addition, such systems require bypasses, afford limited flexibility with changing conditions, and are difficult to maintain. Further, because of the limited flexibility associated with drum screens, sizing requires over-design, and moreover, retrofitting can be costly and difficult. Ice formation and blockage can also be a problem in some cases. Further, the predation requirements of drum screens involves 2 ft/sec. minimum forebay velocities.
Finally the cost tends to be high (approximately $2,000 to $4,000/cfs).
Vertical wedge-wire screen systems suffer disadvantages such as extensive cleaning requirements, and effective cleaning methods are typically mechanical and complex. Further, the submergence requirements of such screens contribute to over-design. In addition, sedimentation is a problem due to low through-screen velocities and debris removal and handling is difficult. The permanent nature of the screen structure limits flexibility, and the length of the structure is extensive, with no water surface elevation control. Further, modification is not easy and as with the other systems discussed above, a by-pass is required and forebay predation a problem.
Small screen systems include the following types: drum screens (paddle wheel or power); vertical wedge-wire; wedge-wire cylinders (air burst or water spray cleaned); wedge-wire ogee; infiltration gallery; perforated plate; and inclined screens. In brief, drum and vertical screens suffer the same problems as above and, in general, submergence requirements can limit installation to deep water. Further, sedimentation fouling can be a problem, and a substantial head loss is associated with some concepts. Also remote cleaning may be ineffective, ice jamming may occur in some cases, and THE CREATION of slack water zones may result in predation. Further, access, inspection and maintenance may be difficult. In addition, the cost of small screen units tends to be high (approximately $4,000 to $20,000/cfs).